Thanks for the replies.
Regarding the use of tags as an alternative. My application is to have each sub-category visible to only a specific group — a private sub-category so to speak. When we’ve set things up this way the only people seeing the long list of sub-categories are the admins. A typical end user would just see their own sub0category and any non-group-specific categories in the system. All other sub-categories would be hidden from them since they aren’t in the connected group.
I’d love to use tags for this instead. Is there a way of attaching a tag to a group and making any topics with that tag only visible to the connected group?